A quartz oscillator of the type which uses a quartz resonator with electrodes that do not adhere to the crystal was described, for example, in French Patent Application No. 2,338,607. Such an oscillator displays many advantages especially with respect to stability over time and mechanical stability. However, as in all categories of oscillators, changes in the ambient temperature induce variations in the frequency of the oscillator which result from changes in the temperature of the vibrating crystal.
In order to neutralize the effects stemming from changes in the ambient temperature, which can evolve on a large scale, for example from -40 degrees C. to +85 degrees C., we must place high stability quartz oscillators inside thermostats. And yet, a thermostat is a voluminous accessory, which consumes energy, and leads to a temperature-setting period that can be as high as one hour for pilot oscillators and very high stability oscillators, to twenty minutes or so with regard to less high performance pilot oscillators.
It turns out that in the realm of quartz oscillators, we are trying more and more to execute devices with reduced volume and mass, which consume less energy, and which display very high equilibrium. Under those circumstances, we must seek means other than thermostats in order to compensate for the effects of ambient temperature on the frequency that is supplied by a quartz oscillator.
Solutions have already been suggested. Among them, the most popular is that which provides electronic compensation for the oscillator frequency, with a variable capacitance diode that is mounted in series with the quartz resonator. With this type of assembly of a quartz oscillator that is temperature-compensated, known as TCXO, the capacitance of the variable capacitance diode is a function of constant voltage V which is derived from static voltage, through a potentiometric network that is comprised of resistors and thermistors. An adjustable compensation voltage is also opposed to the voltage that is output by the potentiometric network. According to a variation of that system, the variable capacitance is achieved by varying the gain from an amplifier, by using the Miller effect according to which the input capacitance of a gain amplifier G can vary like 1-G. This kind of solution is not always satisfactory in the case of high equilibrium oscillators because the implementation of a corrective network which makes use of thermistors to create a control voltage for a variable capacitance diode, or to make the amplifier gain vary, is restricted by the quality of the thermistors used.
Another solution, which was suggested in U.S. Pat. No. 3,826,931, used an algebraic combination of frequencies in which the sum f=f.sub.1 +Kf.sub.2 displays for a given value of constant K a temperature coefficient of the first order that is equal to zero. Frequencies f.sub.1 and f.sub.2 are supplied by two piezoelectric resonators with double rotation which are cut inside two specific regions of a quartz crystal so that the specific orientations .phi. and .theta. of each resonator inside the crystal make it possible to obtain cancellation of temperature coefficients of the 2nd and 3rd order in the algebraic combination which relates f to f.sub.1 and f.sub.2. They finally obtain a frequency f which displays null temperature coefficients for the first three orders of the series development of the frequency-temperature law. However, this solution implies the use of double rotation quartz crystals which are cut in a very specific way in order to achieve compensation for the temperature influence.
A different means of obtaining a frequency with a biresonator that is independent from temperature was recommended by BESSON in the European Patent Application No. 0010046. This solution implements a device which includes two resonators with a single crystalline section of differing frequencies, which are excited within a system of condensors of which a frame is common to both resonators. To the extent that the heat behaviors of the resonators are identical, and by the comparisons which can be made from frequency-temperature curves for each resonator, it is possible to exploit a combination of frequencies for both resonators, this combination being a simple function of temperature. The formation of both resonators with a single crystalline section and identical heat behaviors can be delicate in some instances.
Finally, another form of compensation was recommended in U.S. Pat. Nos. 4,079,280 and 4,160,183. This solution uses a quartz resonator with a section having .phi.=21.93 degrees and .theta.=33.93 degrees, which operates simultaneously on two shearing modes with different thicknesses, modes B and C. A combination of both frequencies is applied to a microprocessor which corrects the frequency error that is due to temperature by way of an analog or digital compensation process. Thus, a reference frequency is artificially re-enacted. In this compensation mode, we use a single vibrating resonator in two different modes, so that its direct couplings may appear between the two vibration frequencies.